Optical bandpass filter using long period gratings

ABSTRACT

The specification describes an optical filter wherein long period gratings (LPGs) are used in a new configuration to provide in-line bandpass filtering of optical signals. In the basic embodiment, two dissimilar LPGs are installed in the optical fiber transmission line. The first LPG converts light, over a broad wavelength range, being transmitted in the fundamental, or LP 01 , mode of the optical fiber transmission line into light in a higher order mode (HOM) of the optical fiber. The mode-converted signal, with mode LP m,n,  is then coupled to a second waveguide, the second waveguide having transmission characteristics different from those of the first. The second waveguide supports propagation of light in a LP m,n  mode. The mode-converted signal is then transmitted through a second LPG where the signal over a selected narrow band of wavelengths that is accepted by the second LPG is converted back to a LP 01  mode. The selected narrow band in the LP 01  mode propagates efficiently over the remainder of the optical fiber transmission path.

RELATED APPLICATIONS

This application claims the benefit of provisional application60/301,164 filed Jun. 27, 2001, which is incorporated herein byreference. This application is a division of Ser. No. 10/167,684 filedJun. 11, 2002.

FIELD OF THE INVENTION

This invention relates to optical filters, and more particularly tooptical bandpass filters with low loss.

BACKGROUND OF THE INVENTION

Optical bandpass filers transmit light over a pre-determined band ofwavelengths while rejecting, by absorption, radiation or scattering, allother wavelengths. Such filters are useful in laser cavities or opticalcommunications systems. For example, they may be used to constrain thewavelength of operation of a laser, when deployed inside or outside thelaser cavity. In optical communications systems, they can be used at theinput of an optical receiver to separate unwanted light such asspontaneous emission noise outside the wavelength band of the signal.See D. M. Shamoon, J. M. H. Elmirghani, R. A. Cryan, “Characterisationof optically preamplified receivers with fibre Bragg grating opticalfibers”, IEEE Colloquium on Optical Fiber Gratings, March 1996. Opticalregenerators based on self-phase modulation require extracting apredetermined wavelength band from a broad spectrum of light.

Several devices have been proposed and demonstrated to offer thefunctionality of bandpass filtering. Fiber Bragg gratings may be used inthe reflection mode with a circulator, or in the transmission mode, toselect a narrow wavelength band. See Xu, M. G.; Alavie, A. T.; Maaskant,R.; Ohn, M. M.; “Tunable fiber bandpass filter based on a linearlychirped fiber Bragg grating for wavelength demultiplexing”, ElectronLeft., 32, pp.1918-1919 (1996). Operation in the reflection moderequires addition of a circulator in the transmission line—thisincreases cost, loss and complexity for the device. A further drawbackwith Bragg gratings, used either in transmission or reflection, is thatsuch filters can be highly dispersive, which gives rise to pulse shapedistortions. See Lenz, G.; Eggleton, B. J.; Giles, C. R.; Madsen, C. K.;Slusher, R. E.; “Dispersive properties of optical filters for WDMsystems”, IEEE Journal of Quantum Electronics, 34, pp.1390-1402, (1998).Alternatively, thin film dielectric filters may also be used as bandpassfilters (see: U.S. Pat. No. 5,615,289), but in addition to theirdispersive nature (similar to Bragg gratings) they suffer from theadditional drawbacks of being free-space devices. Thus, light needs tobe coupled into fiber pigtails for use in such fiber-optic systems. Thisincreases loss, cost and complexity.

An alternative technique for making bandpass filters uses two identicallong-period fiber gratings (LPGs) that are spliced in series in thefiber-optic transmission line, with a core block between them. (See:U.S. Pat. No. 6,151,427) The first long period grating converts a narrowwavelength-band of core-mode light into a cladding mode, and the secondidentical grating couples the cladding-mode light back into the coremode. The core block between the two LPGs attenuates or scatters anylight that was not converted into the cladding mode. There are drawbacksassociated with this device—(1) the core block simultaneously attenuateslight in all the modes, thus the device is inherently lossy.Consequently only higher-order cladding modes may be utilized, as lowerorder cladding modes will exhibit even higher loss; (2) the core blockforms a discrete discontinuity in the fiber, which leads to undesiredmode-coupling and inter-modal interference; (3) tuning such a filterrequires simultaneously tuning both LPGs by identical amounts, as thefilter operates properly only when both LPGs have identical spectra. Inbandpass filters using acousto-optically generated or microbend-inducedLPGs, there is an additional drawback: the device is inherentlypolarization sensitive.

Thus, there exists a need for a bandpass filter that is an in-line fiberdevice, has low loss, is polarization insensitive, tunable, and simpleto implement.

STATEMENT OF THE INVENTION

According to the invention, LPGs are used in a new configuration toprovide in-line band pass filtering of optical signals in an opticalsystem. In a preferred case the optical system is an optical fibersystem. In the basic filter embodiment of the invention, two dissimilarLPGs are installed in serially cascaded optical waveguides of thetransmission line. In the first waveguide of the filter, the signallight is initially transmitted in the fundamental, or LP₀₁ mode. Thefirst LPG converts the signal light, over a broad wavelength range, intoa higher order mode (HOM) of the first optical waveguide. The conversionis achieved using a broadband LPG that provide strong mode conversionover a broad spectrum. See. S. Ramachandran, M. F. Yan, L. C. Cowsar, A.C. Carra, P. Wisk, R. G. Huff and D. Peckham, “Large bandwidth, highlyefficient mode coupling using long-period gratings in dispersiontailored fibers, “Optics Letters, vol. 27, pp. 698-700 (2002); U. S.Pat. No. 6,084,996, both incorporated by reference herein. In thefollowing description the broadband LPG is designated BB-LPG. The BB-LPGoperates by exciting either a core guided HOM or a cladding guided HOMof the first optical waveguide. The mode-converted signal, with modeLP_(m,n), is then coupled to the second optical waveguide, the secondwaveguide having transmission characteristics different from those ofthe first optical waveguide. The second waveguide strongly couples tothe LP_(m,n) mode, because the second LPG, which may be a conventionalLPG, provides a strong narrow band coupling between LP_(m,n) and LP₀₁modes of the second optical waveguide. The second LPG is referred tohere as a narrow band LPG (NB-LPG).

Light entering the dual LPG filter first encounters the BB-LPG wheremore than 99% of the signal is converted to a LP_(m,n) mode over a broadwavelength range. When the converted signal encounters the NB-LPG, thesignal over a selected narrow band of wavelengths accepted by the NB-LPGis converted back to the LP₀₁ mode of the second waveguide. The selectednarrow band in the LP₀₁ mode propagates efficiently over the remainderof the optical waveguide transmission path; i.e., the remainder of thesecond waveguide.

The mode distribution profiles of the LP_(m,n) mode excited in theBB-LPG and the LP_(m,n) converted in the NB-LPG do not have to match, aslong as the order of the modes (m and n) are the same. The BB-LPG andthe NB-LPG may comprise separate waveguide sections, or may be formed insame length of waveguide. The waveguides are preferably optical fibers.If the LPGs comprise separate optical fibers spliced together the signaladiabatically couples at the splice since the mode shapes are the same.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of the bandpass filter of the invention;

FIG. 2 is a plot of grating period vs. wavelength showing therelationship for two LPGs in different optical fibers;

FIG. 3 is a plot of intensity vs. wavelength showing the effective modeconversion properties for the BB-LPG and the NG-LPG;

FIG. 4 is a plot similar to that of FIG. 3 for the filter of FIG. 1,i.e. with the combined LPGs;

FIG. 5 is a schematic diagram illustrating the use of the optical filterof the invention in a laser;

FIG. 6 is a schematic diagram illustrating the use of the optical filterof the invention in a Raman laser;

FIG. 7 is a schematic illustration of the use of the optical filter ofthe invention in an optical signal regenerator device; and

FIG. 8 is a schematic representation of a system using the opticalfilter of the invention in an optical preamplifier for an opticalreceiver.

DETAILED DESCRIPTION

Referring to FIG. 1, the arrangement shown represents the generic formof the bandpass filter of the invention. The filter comprises a firstfiber section 11 with a BB-LPG 12, a second fiber section 13, with aNB-LPG 14, and a splice 15 connecting the two fiber sections together.The splice 15 adiabatically converts the HOM of fiber 11 to the HOM offiber 13. The BB-LPG 12 has a grating period selected for broad-bandmode conversion, and the NB-LPG 14 has a different grating period chosenfor narrow band, i.e., bandpass, mode conversion. The choice of gratingperiods is illustrated in FIG. 2.

With reference to FIG. 2, phase-matching relationships are shown forLPGs coupling the LP₀₁ mode to the LP₀₂ mode (for example). Curve 21represents the phase-matching relationship of a fiber that yields aBB-LPG at about 1470 nm. A unique feature of the curve 21 is theturn-around point (TAP) in the phase matching relationship. At the TAP avertical line 22 is tangent to the phase-matching curve 21. If a gratingperiod has the value corresponding to the tangent line 22, a broad rangeof wavelengths approximately satisfy the phase-matching relationship.For wavelengths in this range, a BB-LPG results, because the wavelengthsare at or close to resonance near the TAP. In this example, a BB-LPGwith a period of 115 microns (line 22) yields a BB-LPG at about 1470 nm.

Curve 23 corresponds to the phase-matching relationship of a LPG in asecond fiber. From curve 23, one sees that the LPG yields a NB-LPG forany wavelength greater than about 1300 nm. That is, for wavelengthsgreater than 1300 nm, a vertical line corresponding to a selectedgrating period is never tangent to the phase-matching curve 23. Thus,there is not a broad range of wavelengths that approximately satisfy thephase-matching relationship. For example, line 24, intersects thephase-matching relationship at only one wavelength.

In this illustration, curve 23 represents a fiber with the same indexprofile as that represented by curve 21, but the fiber associated withcurve 23 is drawn to a diameter that is 80% of the original diameter ofthe fiber associated with curve 21. Dimensionally scaling a fiber inthis manner shifts the TAP of the fiber. Thus, a fiber that yields aBB-LPG when drawn to one diameter, yields a NB-LPG when drawn to adifferent diameter. Dimensionally scaling is one of several ways toshift or adjust the TAP of the fiber. The same objective may be realizedby inducing a constant index change in the fiber, or by etching theouter diameter of the cladding. While the former technique will resultin shifting the TAP of both a cladding mode as well as a core-guidedmode, the latter will be useful when the HOM employed for the bandpassfilter is a cladding mode.

To illustrate the characteristics of the fiber sections 11 and 13 in thefilter of FIG. 1, experimentally obtained spectra are shown in FIG. 3.The phase-matching curve 21 of FIG. 2 for fiber 11 (outer diameter,OD=121 μm) has a TAP at 1540 nm. An LPG written at the correspondinggrating period (112.5 μm) converts the incoming LP₀₁ mode into the LP₀₂mode over the entire C-band. The length of this grating is 1 cm, with anindex perturbation of 5×10⁻³. This illustrates that more than 99% (>20dB) of light is converted over a spectral range between 1527 nm and 1571nm.

More generally, the wavelength range of the BB-LPG may be controlled bysuitably designing a fiber with different dispersion properties for thefundamental mode and the HOM.

The bandwidth, Δλ, of the BB-LPG is given by:Δλ=A×λ _(res) /{square root}{square root over (L×ΔD×)}cwhere ΔD is the difference in dispersion between the two modes that arebeing coupled by the BB-LPG, L is the length of the grating, λ_(res) isthe resonant wavelength (where maximum coupling occurs), c is thevelocity of light in a vacuum, and A is a constant determined by themaximum coupling strength of the grating. Thus, a variety of broadbandspectra may be obtained utilizing this concept. The wavelength range ina typical BB-LPG may range from 40 to 100 nm.

Fiber section 13 is a few-mode fiber similar to fiber 11, but is drawnto an OD of 112 μm. The spectrum of an NB-LPG in this fiber is shown bycurve 32 in FIG. 3. This spectrum is for a 5.7 cm long uniform LPG witha grating period of 120 μm, and an index perturbation of 1×10⁻⁴. The 3dB bandwidth is 7 nm and provides 99.6%. (24 dB) mode-conversion at theresonant wavelength of 1555 nm. The LPG spectra may be tailored inbandwidth by modifying the dispersive properties of the fiber, or thephysical parameters of the grating.

The bandwidth, Δλ, of the NB-LPG is given by:Δλ=A×λ _(res) /L×Δn _(g)where Δn_(g) is the difference in the group indices between the twomodes that are being coupled by the NB-LPG,-and the rest of the termsare as defined earlier. In addition to control over bandwidth, chirpingthe period of the grating, or apodizing the index perturbations of thegratings, can yield a multitude of spectral-shapes (e.g. rectangular,Gaussian, etc.).

With reference to the elements of FIG. 1, the device functions asfollows.

Essentially all light in an entire communications band passing throughBB-LPG 12 is converted to the LP₀₂ mode. As the LP₀₂ mode lighttraverses splice 15, that splices fiber section 11 to fiber section 13,the LP₀₂ mode light of fiber 11 is adiabatically converted into the LP₀₂mode of fiber 13. Then, the NB-LPG 14 selects a desired narrow portionof the spectrum and converts that back into the LP₀₁ mode. This resultsin a bandpass filter, with properties shown by curve 35 in FIG. 4.

The device illustrated in FIG. 1 provides a general platform forbuilding band-selection as opposed to band-rejection filters with LPGs.The first mode-converter, BB-LPG 12, only serves as a device thatprovides a spatially modified input for NB-LPG 14, and does not definethe spectral characteristics of the bandpass filter. The spectrum of thefilter is defined by the inverted spectrum of the NB-LPG 14. Further,instead of only one NB-LPG 14, several NB-LPG may be added in series.Alternatively, the NB-LPG may be designed to possess multiple narrowbandresonances. This enables the prospect of spectral shaping in theband-pass configuration by varying the spectral properties of one ormore NB-LPG 14.

The device just described has an advantage over conventional filtersusing LPGs in that no core-blocking element is used. This avoids theloss inherent in devices with that configuration. For the purpose ofdefining this distinction, the transmission path between the BB-LPG andthe NB-LPG functions adiabatically. Furthermore, the transmission pathcoupling the LPGs does not include an active attenuation element, i.e.an element that has an intended and deliberate function of attenuatinglight. It is not intended that a conventional splice, which is designedfor minimum light attenuation as is not, in this context, an activeattenuation element.

The splice element 15 in FIG. 1 adiabatically transforms the HOM offiber 11 (after traversing the NB-LPG) into a HOM of fiber 13. This maybe achieved by inducing a heat profile along the splice and/or taperingone fiber with respect to the other to ensure that the two HOMs coupleefficiently. In the embodiment shown in FIG. 1, the two fibers 11 and 13are shown with a physical splice joining them, i.e. fibers 11 and 13 areseparate fibers. The physical splice 15, and the attendant loss in thatsplice, may be avoided by making a single fiber with two distincttransmission characteristics. For example, the diameter of the fiber maybe made to vary longitudinally along the fiber length, i.e. a firstsection with a first diameter, and a second section with a seconddiameter. Alternatively, the cladding may be selectively modified fromone portion of a single fiber to another. Therefore, the functionaloperation of splice 15 is the important aspect, and may be defined as ameans for effecting a change from a HOM with one characteristic to acorresponding HOM with a different characteristic.

Since uniform LPGs are not dispersive filters, the bandpass filtersdescribed here are not dispersive.

The filter of the invention may also be tuned by inducing a shift in thephase-matching curve for the NB-LPG (curve 23 of FIG. 2). Alternatively,doping the cladding of the fiber, or coating the outside cladding of thefiber with an electro-optic or non-linear optic material allowselectrical or optical control of the resonant wavelength of the bandpassfilter.

The bandpass filter of the invention may be used in a variety ofsystems. For example, a variety of laser devices can be tuned using thebandpass filter of the invention as an intracavity element. In aconventional laser cavity, defined by two narrowband reflectors, onehigh reflector and one weak reflector (output mirror), the two narrowband reflectors are replaced by two broadband reflectors (one high andone weak) with the bandpass filter of the invention in the cavity. Thelasing wavelength may be adjusted by tuning the filter. Multiple lasingwavelengths can be produced by having one or more NB-LPGS in thebandpass filter that have multiple resonances.

An illustration of this generalized form of laser is shown in FIG. 5,where the gain fiber is shown at 38, high reflector at 39, weakreflector at 41, input signal at 43, laser pump at 44, and WDM forcombining signal and pump at 45. The bandpass filter of the invention,shown generally ar 46, and comprising BB-LPG 47, NB-LPG 48, and splice49, is placed inside the laser cavity as shown.

A specific form of laser, a cascaded Raman fiber laser, is especiallyadapted for use with the filter of the invention. These devices can bemade to operate at multiple wavelengths by employing multiple sets ofBragg gratings that comprise narrowband high-reflectors and outputcouplers/weak reflectors. These pairs of Bragg gratings define lasercavities, and the resonant wavelength of the narrowband grating definesthe lasing wavelength.

An embodiment of this device is illustrated in FIG. 6. The Ramanresonator 51 comprises fiber 52 and gratings 53, 54. In the conventionaldevice, the resonator is bounded on both sides by a narrow band grating.In this embodiment, the resonator is bounded by broadband high-reflector48, and weak reflector 49 gratings. Now, the resonant wavelength iscontrolled by a bandpass filter 56 of the invention. The bandpass filter56 comprises, as described earlier, BB-LPG 57, NB-LPG 58, and splice 59.Tuning is achieved by tuning the NB-LPG of the bandpass filter. SinceLPGs (and therefore the LPG bandpass filters) are widely tunable bymethods described earlier, a swept wavelength source, useful in Ramanamplified optical communications systems, would be easily realizable.For more details on these systems, and for swept wavelengthimplementations in these systems, reference is made to co-pendingapplication Ser. No. 10/098,200 filed Mar. 15, 2002, entitled “WidebandRaman Amplifiers”, which is incorporated herein by reference. Moregenerally, multiple NB-LPG 58 may be added in series to providesimultaneous tunable operation of the device over several lasingwavelengths.

The bandpass filters of the invention are also well adapted for use withself-phase modulation (SPM) based optical regenerators. These devicesare relatively simple and typically comprise a non-linear fiberassociated with a bandpass filter (see U.S. Pat. No. 6,141,129). Thesignal is launched into the non-linear fiber section, thereby inducingSPM in the signal. The effect is to broaden the spectrum of the portionsof the signal that exceed a selected intensity value. The bandpassfilter then selects a predetermined portion of the broadened spectrum.Ideally the bandpass filter for this application should be tunable toaccount for temporal changes in the spectrum. It is also desirable thatthe filter be dispersion-less. These desiderata are satisfied by thebandpass filter of the invention.

A schematic representation of an optical regenerator using the bandpassfilter of the invention is shown in FIG. 7. The non-linear fiber sectionis shown at 62, and the bandpass filter, comprising BB-LPG 67, NB-LPG68, and splice 69, is shown generally at 66.

Pre-amplifiers for optical receivers employ noise filters for removingspontaneous emission noise from the received signal. This noiseoriginates from amplifiers along the transmission path. The noise istypically over the entire spectral range including the signalwavelength, and substantial sidebands of the signal. This backgroundnoise degrades the signal-to-noise ratio of the signal to the receiver.A bandpass filter could selectively attenuate noise outside the signalwavelength, and therefore improve the optical signal-to-noise ratio, butwould be especially effective if the bandpass filter weredispersion-less. That characteristics is provided by the bandpass filterof the invention.

An optical pre-amplifier system as just described is shown in FIG. 8,where the optical amplifier 72 is followed by a bandpass filter, showngenerally at 76, comprising BB-LPG 77, NB-LPG 78, and splice 79. Thefiltered signal is then introduced into a receiver,-represented in thefigure by light sensitive diode 81.

The invention is described above using optical fibers for implementingthe bandpass filter of the invention. Similar devices may be constructedusing other forms of waveguides, for example planar waveguides inoptical integrated circuit (OIC) devices.

The long period gratings described here may be formed by varioustechniques. A common approach is to write the gratings into a Ge dopedfiber using UV light. However, other methods may also be used. Forexample, microbend induced LPGs are suitable. These can be realized withacousto-optic gratings, arc-splicer induced periodic microbends, or bypressing the fiber between corrugated blocks that have the requiredgrating periodicity.

Various additional modifications of this invention will occur to thoseskilled in the art. All deviations from the specific teachings of thisspecification that basically rely on the principles and theirequivalents through which the art has been advanced are properlyconsidered within the scope of the invention as described and claimed.

1. A laser comprising: (a) a resonant cavity, (b) a tuning element inthe resonant cavity, the tuning element comprising: (i) an opticalwaveguide having a first portion and a second portion, (ii) a BB-LPG inthe first portion of optical waveguide, (iii) a NB-LPG in the secondportion of optical waveguide, and (iv) a coupler for coupling the firstportion and the second p[ortion.
 2. The laser of claim 1 wherein thelaser is a Raman fiber laser.
 3. The laser of claim 2 wherein the laseris a Raman cascaded laser.